Modern cellular communication networks typically support numerous user devices, all of which are competing for limited communication resources. Communication service providers face the constant challenge of serving their many customers, many of whose activities consume significant resources, with the infrastructure and communication spectrum available to them. Adding infrastructure to meet increasing demand is costly. In addition, if the spectrum required by the demands of users is greater than the spectrum available to meet those demands, increasing infrastructure will not meet those demands.
To avoid the costs of adding infrastructure, and to help insure that the available spectrum will meet the demands placed upon it, service providers seek to use their available resources as efficiently as possible. One important aspect of efficient resource use is adapting signals to the devices to which they are being transmitted. One mechanism that can be used to adapt signals to devices being transmitted involves the design and use of antenna array structures comprising multiple array elements. A base station, such as an eNodeB (eNB) operating as part of a third generation preferred partnership (3GPP) long-term evolution (LTE) cellular network may comprise an antenna array structure and may coordinate signals using the array structure in order to efficiently direct a transmission. LTE network standards define a number of codebooks to reduce the signaling required between an eNB and a user equipment (UE). A codebook is simply a collection of vectors or matrices which describe the phasing of the eNB's transmit antenna array. Each entry (row) in a vector or matrix describes the corresponding phasing of eNB's port number corresponding to the row. In 3GPP, which port number maps to a particular physical antenna is implementation specific at an eNB. However as will be described, certain antenna array configurations (such as an array of 8 antennas made up of four cross-polarized antennas aligned in azimuth) have an implicit port to physical antenna mapping described in the standard 3GPP. A number of codebooks are defined, including codebooks designed for eNBs with specified numbers of transmit antennas. For example, an eNB may use a particular codebook based on the number of transmit antennas it employs: a 2-antenna eNB may use a 2-antenna codebook, for example, or an 8-antenna eNB may use an 8-antenna codebook. A UE performs feedback of information such as a precoder matrix index, rank indicator, and channel quality indicator (CQI) to an eNB. The UE spatial feedback for a subband represents a precoder, and CQI is computed based on the assumption that the eNB uses a specific precoder indicated by the feedback, on each subband within the CQI reference resource.
The eNB defines transmission to a UE based on the feedback received from the UE, and aspects of the transmission are defined by a codebook shared by the eNB and the UE. The codebook comprises a number of indexed lookup tables, so that information may be transferred between an eNB and a UE by transmitting an index, rather than the value represented by the index.
One mechanism used in cellular networks such as LTE cellular systems is beamforming, in which signal components are directed to elements of an antenna array so as to direct a transmission beam to a particular UE. One use of codebooks) is to allow for the definition of beamforming vectors, which provide information indicating the antenna array elements to be used for transmission to a UE and the weighting of signal components directed to each transmission element. A UE may select from its codebook the best beamforming matrix and may convey the selection over a feedback channel to the eNB. An index of the selected beamforming vector or matrix is fed back to the eNB, which then looks up the corresponding codebook and selects the beamforming vector or matrix indicated by the index.